For the last five decades, diet soda has been a given for anyone on a diet who wants to drink something other than water. This because instead of it containing large quantities of sugar it had artificial sweeteners, which according to physicians and dietitians would not make you put on more weight than if you drank regular water.

I realize this is, as a whole, is still the popular belief. But every now and then you see news articles or TV segments that hint that these sweeteners actually don't help you lose weight at all.

I'm looking for some concrete factual evidence on this. If the old belief is true, then fine. But if it isn't then what could be reason? Is it that the consumers often combine their diet soda with eating something that is unhealthy and fattening? Or is it in the actual sweetener, that it in itself is what makes people put on weight?

@Oliver_C Thanks for the links! Ok, so it's up for debate even on a researching level then. As mentioned in the article I've also heard about these accusations of artificial sweeteners causing cancer, however I don't remember it as being bladder cancer (as mentioned in the article) but rather brain cancer occurring from tricking the brain that you're adding sugar to the body, and that the brain adjusts itself to that. And when no sugar comes the brain can take damage. In any case, thanks.
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AndroidHustleDec 10 '12 at 12:21

diet soda has been a given for anyone on a diet who wants to drink something other than water What's wrong with tea, herbal tea, coffee, and drinks that just have less sugar than normal soda (some juices, some sodas, etc.)?
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Brendan LongDec 11 '12 at 23:27

@BrendanLong when imbibing along with a meal, I for one prefer a drink of the cooling kind. And juices, in general, contain large quantities of both fructose and sucrose as well.
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AndroidHustleDec 12 '12 at 0:20

2 Answers
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I suspect that no definitive answer is known, but below is some interesting research. In general, it sounds like little research has been done to directly test the question humans. Epidemiological evidence suggest otherwise, though that may be not be causation (ie overweight people trying to stem weight gain might tend to eat artificial sweeteners more). If the food reward hypothesis of obesity is correct, that would also suggest that artificial sweeteners wouldn't necessarily aid in weight loss if they increase the palatability of caloric food.

Surprisingly, epidemiologic data suggest the contrary. Several large scale prospective cohort studies found positive correlation between artificial sweetener use and weight gain. The San Antonio Heart Study examined 3,682 adults over a seven- to eight-year period in the 1980s. When matched for initial body mass index (BMI), gender, ethnicity, and diet, drinkers of artificially sweetened beverages consistently had higher BMIs at the follow-up, with dose dependence on the amount of consumption. Average BMI gain was +1.01 kg/m2 for control and 1.78 kg/m2 for people in the third quartile for artificially sweetened beverage consumption. The American Cancer Society study conducted in early 1980s included 78,694 women who were highly homogenous with regard to age, ethnicity, socioeconomic status, and lack of preexisting conditions. At one-year follow-up, 2.7 percent to 7.1 percent more regular artificial sweetener users gained weight compared to non-users matched by initial weight. The difference in the amount gained between the two groups was less than two pounds, albeit statistically significant. Saccharin use was also associated with eight-year weight gain in 31,940 women from the Nurses’ Health Study conducted in the 1970s.

Similar observations have been reported in children. [...] In addition, consensus from interventional studies suggests that artificial sweeteners do not help reduce weight when used alone. BMI did not decrease after 25 weeks of substituting diet beverages for sugar-sweetened beverages in 103 adolescents in a randomized controlled trial, except among the heaviest participants. A double blind study subjected 55 overweight youth to 13 weeks of a 1,000 Kcal diet accompanied by daily capsules of aspartame or lactose placebo. Both groups lost weight, and the difference was not significant. Weight loss was attributed to caloric restriction. Similar results were reported for a 12-week, 1,500 Kcal program using either regular or diet soda. Interestingly, when sugar was covertly switched to aspartame in a metabolic ward, a 25 percent immediate reduction in energy intake was achieved. Conversely, knowingly ingesting aspartame was associated with increased overall energy intake, suggesting overcompensation for the expected caloric reduction. Vigilant monitoring, caloric restriction, and exercise were likely involved in the weight loss seen in multidisciplinary programs that included artificial sweeteners.

Edit: July 7, 2013: This opinion/summary piece from in Cell gives a good summary. Based on the research discussed in the paper, the answer to your original question is: the best available evidence points to yes, artificial sweeteners can make you gain weight.

Fructose is found in fruits and in cane sugar but there seems to be different effects caused by what its ingested with and the concentration. Personally I'm trying to avoid HFCS but its getting harder since fructose is sweeter and costs less.

Cite, please. AIUI, diet sodas are in fact very unlikely to have HFCS in them, because of its calorie content. The sweetener is much more likely to be aspartame these days.
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Kaz DragonDec 12 '12 at 7:50

@Kaz The talk is actually incredibly well referenced. Just posting a (quite long!) video is probably not very useful as a sole answer, however. Personally I find the arguments and evidence marshalled in this talk quite solid. If OP could write this up in a short paragraph with the references mentioned in the video that woul d be awesome. – But yeah, the point about diet sodas is different, and I’m not actually sure that the video supports this at all.
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Konrad Rudolph♦Feb 19 '13 at 16:36

The talk is indeed well referenced, but it doesn't claim that Diet soda contains HFCS. And it would be wrong if it did. Diet drinks are usually calorie free because they are sweetened by aspartame, saccharin or similar compounds which are so much sweeter than sucrose or HFCS that their calorie contribution is negligible.
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matt_blackJan 3 '14 at 18:47